The present invention relates to method and apparatus for establishing multiple, selectable position ranges within which a positionable airfoil of an aircraft is to be disposed for assuring controllability of the aircraft under certain predetermined flight conditions.
On certain existing aircraft, it is standard practice to designate a predetermined range of acceptable positions within which a movably mounted stabilizer airfoil is to be disposed prior to takeoff. The position of the stabilizer is referred to as the "trim". The acceptable range of trim settings may be calculated from the weight and balance of the aircraft, which refers to the gross weight and location of the center of gravity respectively. Although the gross weight and center of gravity will vary depending upon the load carried by the airplane, these variables can be taken into account in establishing the acceptable limits of the stabilizer position range. This permissible range is sometimes referred to as the "green band", inasmuch as a band of green paint is typically provided adjacent the stabilizer position indicator to define the position limits of the predetermined range.
Associated with the "green band" is a warning system, including position sensors in the form of limit switches disposed to be actuated by the movement of the stabilizer. By connecting the limit switches to actuate an alarm or other warning indicator, and disposing the switches at positions matching the limits of the "green band" on the stabilizer indicators, a means is provided for warning the pilot when the stabilizer position exceeds the predetermined limits. So long as the stabilizer is positioned by the pilot within the applicable "green band", takeoff controllability is assured for any gross weight and/or center of gravity loading for which the aircraft has been certified.
Although the single "green band" range has been found satisfactory for many aircraft, larger airplanes having higher thrust engines and/or greater gross weight load capacity, exhibit reduced controllability for stabilizer positions within the previously utilized range. In order to maintain the same level of controllability, or to improve upon existing controllability, it has been found necessary to narrow the "green band" range. The narrower, single range "green band" in turn requires either a change in the permissible gross weight and center of gravity loading limits of the aircraft, or redesign of the aircraft elevator for increased pitch control authority. Neither of these alternatives is desirable.